RNA Ligase, an enzyme catalyzing the formation of an internal phosphodiester bond at the 5'-terminus of polyribonucleotides has recently been described by the principal investigator and associates in E. coli infected with T-even bacteriophage (R. Silber, V.G. Malathi, and J. Hurwitz, Proc. Nat. Acad. Sci., 69:3009, 1972). Preliminary data suggest that this enzyme is also found in certain mammalian cell lines. In the proposal detailed below, we plan to purify and study this activity in a variety of mammalian systems. The purified enzyme will be characterized as to cofactor and substrate requirements. The reaction product will be analyzed to determine if an intra- or intermolecular joining reaction has taken place. After these aspects are clarified, the effect of several other factors on RNA ligase will be determined: These include infection with RNA viruses, cell maturation and a possible relationship to replication or recombination of oncogenic RNA modification or repair will be evaluated. The levels of RNA ligase will be determined in leukocytes in normal subjects and patients with leukemia. The study is entirely in vitro and humans will be used solely as a source of blood cells, with the only procedure involving them being venipuncture.